An image sensor is a semiconductor device for converting an optical image into an electric signal. The image sensor may be roughly classified into a charge coupled device (CCD) image sensor and a complementary metal oxide silicon (CMOS) image sensor (CIS).
A CIS generally includes a photodiode and at least one MOS transistor formed in a unit pixel, and obtains an image by sequentially detecting electrical signals of unit pixels in a switching manner.
In a related art CIS structure, a photodiode and a transistor are horizontally arranged.
Although the related art horizontal-type CIS has solved many limitations of CCD image sensors, the horizontal-type CIS still has several problems.
First, the horizontal-type CIS of the related art typically has a photodiode and a transistor that are horizontally formed adjacent to each other on a substrate. Therefore, an additional region for forming the photodiode is required, which may decrease the fill factor and limit the possibility of resolution.
Also, in the horizontal-type CIS of the related art, it is generally very difficult to achieve an optimized process of concurrently forming the photodiode and the transistor.